Time Dependent Changes in Notch Stress/Notch Strain and Their Effects on Crack Initiation

Abstract

An analytical and experimental program was conducted to evaluate the time-dependent changes in the stress-strain state at stress concentrations. Load-time interaction effects on creep and stress relaxation were evaluated utilizing simple coupon, super-scale, and simplified stress concentration test specimens. Periods of sustained compression loads included in a load sequence or spectrum were shown to affect a reduction in specimen fatigue life due to creep and stress relaxation occurring during the hold period. Experiments were designed such that a quantitative assessment of time-dependent changes in both notch stress and notch strain was possible. Resulting data were used to formulate a creep and stress relaxation module for inclusion in an automated hysteresis fatigue analysis program. Agreement between experimental and predicted lives using the hysteresis analysis is significantly better than predictions using a linear analysis method.